"It's what I call the disease of collecting," retired Chicago businessman Steve Rineberg says. "People collect baseball cards, too. But there are not too many baseball-card museums."

Rineberg and his wife, Gail, have collected contemporary art and Asian ceramics for years and are frequent donors to the Phoenix Art Museum, which is celebrating its 50th anniversary with a new installation, "50/50: Fifty Gifts for Fifty Years."

That is, there are 50 new artworks - or collections of artworks - given by 50 donors over the past year to celebrate the museum's anniversary.

"A golden anniversary provides a golden moment to reflect on the successes of our institution," says James Ballinger, museum director.

It's also a moment to reflect on the importance of donors to the growth of the institution.

Beside every painting in the Phoenix Art Museum is a little white tag that visitors read; it gives the title of the work and the name of the artist. The tag also tells a story about the painting itself and its relationship to the museum and the city. It tells when the museum obtained the artwork, and how. It provides a subtext to a visit.

It isn't just the art, but the story the painting tells, who saw it in a gallery or at auction, loved it, bought it and, finally, gave it to the museum.

'Zero' buying budget Few visitors probably give a thought to how that art found its way into the museum. The museum buys art, right?

"Our purchase budget each year is zero," Ballinger says. "It always has been zero."

That means all of the 18,000 pieces in the museum's holdings come from an outside source, or the funding does.

On each little white tag, at the end of the text, you will see a code number, such as 1983.89, which means the Gilbert Stuart portrait of George Washington was acquired in 1983 and was the 89th piece acquired that year.

The tag also includes the name of the donor who gave the work, in this case, Gilbert A. Harrison.

As you walk through the museum, you see many names over and over: Sybil Harrington, Orme Lewis, Bud Jacobson. These are people who have supported the museum with their wallets as well as their time.

"Orme Lewis is the biggest donor, over the long haul," Ballinger says.

"He gave us over a thousand works of art."

Lewis, who died in 1990, was co-founder of the prominent law firm Lewis and Roca.

"For many years, he was a huge print collector, collecting for his office, for the museum and for himself. And when things got crowded, he packaged up the overflow and sent them to the art museum.

"I was in his house one day and saw a Marsden Hartley painting sitting in the bathtub of his guest room. I said, 'Orme, you're out of space in your bathtub. You've got to make more room,' and he said, 'Oh, if you want that, take it.' "

That giveaway now is on loan to the national traveling show "Cezanne and American Modernism."

Every painting on the museum walls tells a similar story: Someone owned this work and thought enough to give it to the museum so we can all share it.

"We buy to share," Rineberg says. "We complete a collection and then give it to the museum so the public can enjoy it."

'Boils down to people' Ballinger says there are differing motives for giving.

"There are basically three ways for the museum to get art," he says. "First, people donate works; second, we have support groups who raise money and purchase works of art in their area; or third, we have somebody give money and we go out and spend it."

The support groups, such as the Men's Arts Council, the Contemporary Forum and the Arizona Costume Institute, each highlight a separate area of the museum's collection, and its members raise funds to buy art in that area. They hold fundraising events throughout the year, like the Men's Arts Council's Copperstate 1000 motor rally and their Cowboy Artists of America exhibition and sale.

"It all boils down to people," says Ellen Katz, chairman of the board of trustees for the Phoenix Art Museum.

Katz and her husband, Howard, are long-time supporters of the museum. The new wing of the museum, where contemporary art is shown to advantage, is named for them, after their generosity.

That generosity stems from three primary motives, Ballinger says.

"Many collectors, after a period of years, after building a significant collection, will feel a sense of responsibility to share what they have with the community," he says. "A work of art has a life that goes well beyond them, and they are the proud custodians of that painting or sculpture, and they feel a responsibility to place it in the public trust."

Those are collectors whose initial loyalty is to the art itself.

"Then," Ballinger says, "there are collectors that feel a responsibility to the museum directly, to maintain it for the community and make gifts for that reason."

They often will collect in an area they know the museum needs to grow, and they work with museum personnel to decide what those needs are.

"Finally," Ballinger says, "there are those who frankly need or want a tax deduction in certain years. If they get the tax advantage and are doing good at the same time, that's fine, too."

Mostly, he says, you see blends of the three motives.

The Rinebergs, for instance, will consult with Janet Baker, the Asian-art curator, about items the museum might need to complete an area of study, say, 17th-century celadon pottery from China, and then study the field, buy significant pieces of that art and donate them to the museum.

"Steve knows more about Chinese ceramics than anyone I know who does not speak Chinese," Baker says.

Steve Rineberg uses his expertise to help the museum, which he speaks about with a proprietary "we."

"Go to other museums," he says, "and compare what we have. We tell the story of Chinese ceramics from the Neolithic right through the 1700s."

'From the heart' Most collectors have the money to afford their habit.

"But money is a relative thing," Ballinger says.

Not every museum donor is a retired businessman.

"Oftentimes, you'd be surprised," says Jerry Smith, the museum's associate curator of American art. "They might be well off, but not really wealthy. It's not always the Paradise Valley crowd. They might be living in a simple tract home but always had that bug for collecting."

He tells the story of Western-art expert Ginger Renner, who, when she was young and just beginning, found a Nicolai Fechin painting she bought for $50 down and $50 a month for a year.

"It's now worth many times that," Smith says.

Bud Jacobson, who died in 2005 at 82, began collecting when he was a law clerk for the Arizona Supreme Court more than a half-century ago. He bought four Picasso etchings for $450. At the time, he was earning $256 a month.

"It took me quite awhile to pay it off," he said in 2003.

Gene Koeneman is chief preparator for the museum. He builds installations, packs and ships art, and handles the physical aspects of hanging and displaying artwork.

With his wife, Sherry Beadles, he has amassed a collection of Japanese woodblock prints from the late-19th and early-20th centuries. Such prints are not outrageously expensive. You might find one for sale for anywhere from 40 to several hundred dollars. But if you buy with a purpose in mind, you wind up with a significant collection, like theirs.

They have donated 23 prints by the artist Tsukioka Kogyo, and an intact album of 52 more prints. Kogyo specialized in images from Japanese Noh theater, and what the couple have given to the museum enhances the museum's holdings in this area of art history.

"We collect because we enjoy the objects," Koeneman says. "I hope that's why people do it, in general.

"Former curator Claudia Brown showed us some Japanese prints, and we took a liking to them. One day, my wife said I had to go look at something she found in a thrift store, and, after that, we just kept getting more."

Most of their prints were made from 1850 to 1950, a period that had not been much sought-after, but because Koeneman and Beadles bought judiciously, their collection now is valuable.

"I work at the museum to make a living. I can't collect a lot of things, but I can collect prints," he says.

Collectors, then, do it not merely as an investment, but because they love the work.

"Collectors are passionate people," Katz says. "It comes from the heart. It's an emotional thing, not an analytical thing.

"We have the ability to make a difference to the arts and culture in the Valley, so why not do it?"